Liquid crystal displays (LCDs) are commonly used in cell phones, personal digital assistants, laptop computers, desktop monitors, and televisions. One embodiment of the present invention deals with a color, transmissive LCD that requires white light backlighting.
FIG. 1 is a cross-sectional view of a prior art color, transmissive LCD 10 that includes a backlight 12 using LEDs. The backlight contains an array of red, green, and blue LEDs 14 whose combined light forms white light.
The backlight 12 ideally provides homogenous light to the back surface of the display. The backlight box has reflective bottom and side walls to mix the red, green, and blue light. The inner surfaces may be painted white. Mixing optics 16, such as a diffuser, improves the color mixing.
Above the mixing optics 16 are conventional LCD layers 18, typically consisting of a first polarizer, a thin film transistor array layer, a liquid crystal layer, a ground plane layer, a second polarizer, and RGB filters. Each red, green, and blue subpixel on the LCD screen is formed by the white light transmitted through the second polarizer in that subpixel area being filtered by a corresponding red, green, or blue filter portion. A single pixel of an image is formed by a set of red, green, and blue subpixels. The electric fields created at each subpixel location, by selectively energizing the thin film transistors at each subpixel location, causes the liquid crystal layer to change the polarization of the white light at each subpixel location. By controlling the thin film transistors, the magnitude of white light being filtered at each red, green, and blue subpixel is controlled to create an image on the LCD screen. LCDs are well known and need not be further described.
Using red, green, and blue components for the white light is particularly advantageous in a backlight because the RGB emission wavelengths correspond well to the spectral distributions of the RGB color filters.
The problem with the above backlight is that, since the red, green, and blue LEDs are separated, it is very difficult to obtain color uniformity across the backlight. Deep backlight boxes can be used to improve color mixing and/or special lenses on the LEDs may be used; however, such solutions become expensive and add to the size of the LCD.
White light LEDs are known and can be used in a backlight box. With ideal white light LEDs, there is only the need for brightness uniformity across the LCD screen, which is relatively simple to achieve.
One type of white light LED uses a blue LED with a YAG phosphor coating the LED. The YAG phosphor primarily emits yellow-green light when energized with the blue LED. The YAG phosphor emits a broad range of wavelengths including some red. Some of the blue light is transmitted through the YAG phosphor and combines with the yellow-green light to create white light. The YAG phosphor powder may be mixed in a liquid binder and deposited on the blue LED. The binder is then cured. One problem with such a white light LED is that the blue and yellow-green color components do not match well with the RGB filters in an LCD, resulting in low color gamut and low light transmission out of the LCD. For example, the red component of the white light is low compared to the green and blue components. Another problem with this type of white light LED is that the thickness and density of the YAG phosphor varies across the surface of the blue LED, resulting in nonuniform color.
Another problem with white light LEDs, is that the blue LEDs have variations in wavelength and brightness due to production tolerances. As a result, the color points of white light LEDs vary even when the phosphor coating is perfectly even.
It is known to affix a YAG phosphor plate over a blue LED die. However, due to the variations in the spectral distribution of the blue LED, the white color point varies. Hence, any backlight incorporating the white light LEDs will not have color uniformity across the screen.
It is also known to deposit red and green phosphors over a blue LED, where the blue light leaks through the phosphors to create white light. However, it is very difficult to control the magnitudes of the RGB emission components. The red and green phosphors coating the blue LED may vary in thickness and density across the blue LED, and may vary in thickness and density from one LED to another, resulting in color nonuniformity. The red and green phosphors may be deposited in a liquid binder, then cured, or deposited using other techniques such as electrophoresis.
What is needed is a white light LED that provides RGB components with a highly repeatable color point and which emits a uniform white light.